Polypropylene is a well known commercial polymer, used for a variety of products such as packaging films and extruded and molded shapes. It is produced by polymerization of propylene over transition metal coordination catalysts, specifically titanium halide containing catalysts. Commerical polypropylene is deficient in resistance to impact at low temperatures, i.e., 0.degree. C. and below. It is known that incorporation of some elastomers, particularly elastomeric copolymers of ethylene and propylene, improves the low temperature impact resistance of polypropylene.
One method of incorporating elastomeric ethylene-propylene copolymers into polypropylene is by sequential polymerization of propylene and ethylene-propylene mixtures. In typical processes of this kind, propylene homopolymer is formed in one stage and the copolymer is formed in a separate stage, in the presence of the homopolymer and of the original catalyst. Multiple stage processes of this type are also known. Products of such sequential polymerization processes are sometimes referred to as "block copolymers" but it is now understood that such products may rather be intimate blends of polypropylene and ethylene-propylene elastomer. The products of such sequential polymerization of propylene and ethylene-propylene mixtures, are referred to herein as sequentially polymerized propylene-ethylene copolymers or as in-situ produced copolymers. To maintain separate terminology for the total sequentially polymerized copolymer composition and the elastomeric copolymer fraction thereof, the total copolymer composition is referred to as impact-improved propylene-ethylene copolymer which has a specified content of an elastomeric ethylene-propylene copolymer fraction and which is the product of sequential polymerization of propylene and a propylene-ethylene mixture.
Methods for producing impact-improved, sequentially polymerized propylene-ethylene copolymers are well known. See, for example, "Toughened Plastics" by C. B. Bucknall, Applied Science Publishers Ltd. 1977, pp. 87-90, and T. G. Heggs in Block Copolymers, D. C. Allport and W. H. James (eds), Applied Science Publishers Ltd. 1973, chapter 4. Representative U.S. patents describing such methods are: U.S. Pat. Nos. 3,200,173-- Schilling; 3,318,976-- Short; and 3,514,501-- Leibson et al.
As disclosed in U.S. Pat. No. 3,514,501, a propylene polymer preblock is prepared, preferably in the liquid phase, by catalytic polymerization of propylene in a hydrocarbon diluent such as liquid propylene to form a slurry. After a separation of the slurry, the prepolymer which still contains active catalyst residues is introduced into at least one reaction zone, where it is reacted with monomer vapors for a sufficient period of time to form the polymer post block onto the polymer preblock in the desired proportions.
In the past, the conventional catalyst system used in such a polymerization process has been an unmodified or an electron donor-modified titanium halide component, activated with an organoaluminum cocatalyst. Typical examples of conventional propylene polymerization catalyst systems include cocrystallized titanium trichloride-aluminum trichloride catalysts of the general formula n.TiCl.sub.3 AlCl.sub.3 activated with diethylaluminum chloride or triethyl aluminum. The cocrystallized titanium trichloride-aluminum trichloride may also be subjected to a modification treatment with a suitable electron donor compound to increase its activity or stereospecificity. Such compounds include phosphorus compounds, esters of inorganic and organic acid ethers and numerous other compounds.
One major drawback, however, in using the aforementioned conventional catalysts, has been the low catalyst productivity, which has necessitated the subsequent deashing of the product to reduce the content of catalyst residues, which otherwise would detrimentally affect the product quality.
Recently new catalysts have been developed which are far more active than the aforementioned conventional catalysts in the polymerization of alpha-olefins. These olefin polymerization catalysts are prepared by combining a solid component comprising at least magnesium, titanium and chlorine with an activating organoaluminum compound. These may be referred to as supported coordination catalysts or catalyst systems. The activity and stereospecific performance of such compositions is generally improved by incorporating an electron donor (Lewis base) in the solid component and by employing as a third catalyst component an electron donor which may be complexed in whole or in part with the activating organoaluminum compound.
For convenience of reference, the solid titanium-containing constituent of such catalysts is referred to herein as "procatalyst", the organoaluminum compound, whether used separately or partially or totally complexed with an electron donor, as "cocatalyst", and the electron donor compound, whether used separately or partially or totally complexed with the organoaluminum compound, as "selectivity control agent" (SCA).
Supported coordination catalyst of this type are disclosed in numerous patents. The catalyst systems of this type which have been disclosed in the prior art generally are able to produce olefin polymers in high yield and, in the case of catalysts for polymerization of propylene or higher alpha-olefins, with high selectivity to stereoregular polymer. However, further improvements in productivity at high stereoregularity are still being sought.
The objective of workers in this art is to provide catalyst systems which exhibit sufficiently high activity to permit the production of polyolefins in such high yield as to obviate the necessity of extracting residual catalyst components in a deashing step. In the case of propylene and higher olefins, an equally important objective is to provide catalyst systems of sufficiently high selectivity toward isotactic or otherwise stereoregular products to obviate the necessity of extracting atactic polymer components.
Although many chemical combinations provide active catalyst systems, practical considerations have led the workers in the art to concentrate on certain preferred components. The procatalysts typically comprise magnesium chloride, titanium chloride, generally in tetravalent form, and as electron donor an aromatic ester such as ethyl benzoate or ethyl-p-toluate. The cocatalyst typically is an aluminum tiralkyl such as aluminum triethyl or aluminum tri-isobutyl, often used at least partially complexed with selectivity control agent. The selectivity control agent typically is an aromatic ester such as ethyl-para-methoxy-benzoate (ethyl anisate) or methyl-p-toluate.
The productivities obtained with these new catalysts are extremely high resulting in polymers containing such small quantities of residual catalyst that the conventional deashing step can be dispensed with. The catalysts function well in the homopolymerization of propylene and in the copolymerization of a mixture of propylene and another alpha-olefin such as ethylene, provided that the polymerization reaction is carried out in a liquid diluent, e.g., liquid propylene monomer. However, in the vapor phase polymerization used in preparing the EP copolymer block of P-EP block copolymer described above, using conventional operating conditions, it has been found that the rapid catalyst deactivation left insufficient catalyst activity in the gasphase copolymerization reactors.
One means to improve the activity in the copolymerization is disclosed in U.S. Pat. No. 4,284,738. In the '738 patent, vapor phase polymerization is conducted in the presence of further added quantities of aluminum trialkyl catalyst components amounting from about 5 to about 50% of the quantity used in the preparation of the propylene prepolymer. Increases in ethylene incorporation as well as in impact strength of the product are achieved by this process improvement. However, these are still some problems connected with the aforementioned process in that process control is somewhat difficult to maintain. For instance, heating and cooling requirements in the vapor phase reaction zone fluctuate considerably, and if not constantly monitored, can cause unwanted variations in polymerization temperature, productivity, and product quality. It sometimes results in a "sticky" polymer product with attending materials handling problems.
Another approach is disclosed in U.S. Pat. No. 4,334,041. In the '041 patent, additional quantities of supported titanium halide catalyst component are added to the vapor phase reaction zone and the alkyl aluminum catalyst compound added to the vapor phase reaction zone is at least partially complexed with an electron donor compound at a mole ratio of alkyl aluminum to electron donor within a very narrow range and different from that used in the prepolymer preparation. It is also important in the patent that the temperatures of each of the zones be controlled within rather narrow limits or the extent of the above described improvements will be less or even completely counteractive.
A new process has now been found that results in high yields of impact copolymers without the problems associated with the '738 and '041 processes, and which has the added benefit of reducing the formation of "gels".